How does Down syndrome occur

Down syndrome can occur when a portion of chromosome 21 becomes attached (translocated) onto another chromosome, before or at conception. These children have the usual two copies of chromosome 21, but they also have additional genetic material from chromosome 21 attached to another chromosome.

usually caused by an error in cell division called nondisjunction.  babies are normally born with 46 chromosomes, but when they have down syndrome they have an extra copy of one of the chromosomes

Explanation:

cytokinesis is the splitting of a cell membrane to form 2 identical daughter cells. it's similar to mitosis as both cells have identical copies of dna and cytokinesis is a step in the mitotic cycle.